1. Field of the Invention
The present invention relates to a fine particle structure suitable for a photonic crystal layer, and an optical medium suitable for a reflective type screen suitable for projection of an image from a CRT (cathode ray tube) projector or a liquid crystal display projector.
2. Description of Related Art
As a reflective type screen for displaying an image by reflecting light projected from a projector, a white screen has been conventionally used that reflects or scatters whole light within the visible wavelength region without wavelength characteristics. In the case where light having no relation to an image is incident on a white screen, the light is reflected or scattered as similar to the image (hereinafter, such visible light other than an image projected from a projector that is incident on the screen irrelatively to the image is refereed to as external light). As a result, the external light is viewed by an observer along with the image to deteriorate the contrast of the image.
Accordingly, an image is projected from a projector onto a white screen generally in a dark room where entrance of external light is restricted. However, the fact that the image display is restricted to a dark room significantly impairs usefulness of the display system using the screen and considerably restricts the application thereof. Even in the case where an image is projected in a dark room, the contrast of the image is lowered due to external light, such as light reflected by the screen that is scattered in the dark room and is again incident on the screen, light invading from the outside, and light remaining in the dark room including emergency light, and therefore, a dark area in an image cannot be displayed as a true dark area on a screen.
Some projectors among CRT projectors and liquid crystal display projectors display various colors by projecting three primary colors of red (R), green (G) and blue (B) onto a screen and mixing the colors on the screen. Projectors of the type are restricted in reproducible color area on the chromaticity diagram due to a broad spectral full width at half maximum (FWHM) of the three primary colors of from 60 to 100 nm, and thus suffers from such a problem that accurate color tone is difficult to be reproduced.
As a result of earnest investigations on the aforementioned problem, the inventors proposed a screen having such wavelength characteristics that visible light at particular wavelengths is reflected, a process for producing the same, and an image displaying system using the screen (Japanese Patent Application No. 2001-380670). In a screen based on a preferred embodiment of the invention described in Japanese Patent Application No. 2001-380670 (hereinafter, referred to as the prior invention), a reflective layer that selectively reflects only light having narrow wavelength areas in the vicinities of the three primary colors in the visible region is provided on the surface of the screen, and an absorbing layer that absorbs visible light passing through the reflective layer is provided under the reflective layer in the thickness direction.
Upon displaying an image on the screen, light of the three primary colors, red (R), green (G) and blue (B), is projected from a projector onto the screen, and an image having various colors is formed by color mixing on the screen. The light of the three primary colors is reflected by the reflective layer provided on the screen to reach eyes of a viewer, and thus perceived as an image.
The external light contains light having various wavelengths, and the most part thereof is light outside the wavelength regions in the vicinities of light of the three primary colors, which can be reflected by the reflective layer. Therefore, even in the case where the external light is incident on the screen, the most part thereof is not reflected by the reflective layer but is absorbed by the absorbing layer, whereby the external light substantially does not reach eyes of a viewer along with the image. As a result, deterioration in contrast due to the external light is significantly suppressed, and therefore, a dark area in an image can be displayed as a true dark area on the screen. Furthermore, an image can be displayed on the screen outside a dark room, for example, in a bright room and out of doors.
The screen functions as a filter for selecting light depending on wavelength by itself according to the aforementioned mechanisms, and accordingly, the color reproducibility of an image is also improved owing to the following reasons. The spectral full width at half maximum of light of the three primary colors emitted from a CRT projector and a liquid crystal display projector is broad, as having been described. However, when the light is incident on the screen, only light of the three primary colors and the narrow wavelength regions in the vicinities thereof is selectively reflected, but other light is wholly absorbed. Consequently, the light of the three primary colors thus reflected by the screen is improved in color purity with a narrow spectral full width at half maximum. Therefore, the reproducible color area of an image formed by mixing the light of the three primary colors thus reflected is enhanced, and the color tone is more accurately reproduced.
FIG. 12 is a schematic cross sectional view for showing the principal of selection of an image and external light by a reflective type screen 30 according to a preferred embodiment of the prior invention. The reflective type screen 30 has a reflective layer 32 that selectively reflects light having narrow wavelength regions in the vicinities of light of the three primary colors, red (R), green (G) and blue (B), in the visible region, and an absorbing layer 31 for absorbing visible light passing through the reflective layer under the reflective layer in the thickness direction.
Upon displaying an image on the screen 30, light of the three primary colors, red (R), green (G) and blue (B) from a projector etc., is projected onto the screen, and an image having various colors is formed by color mixing on the screen. The light of the three primary colors is reflected by the reflective layer 32 to reach eyes of a viewer, and thus perceived as an image.
The external light contains light having various wavelengths, and the most part thereof is light outside the wavelength regions in the vicinities of the three primary colors, which can be reflected by the reflective layer 32. Therefore, even in the case where the external light is incident on the screen 30, the most part thereof is not reflected by the reflective layer 32 but is absorbed by the absorbing layer 31, whereby the external light less reaches eyes of a viewer along with the image. As a result, deterioration in contrast due to the external light is significantly suppressed, and therefore, clearness of an image is improved and a dark area in an image can be displayed as a true dark area on the screen. Furthermore, an image can be displayed on the screen outside a dark room, for example, in a bright room and out of doors.
FIGS. 13A and 13B are schematic cross sectional views showing specific examples of the structure of the reflective layer that selectively reflects light having a particular wavelength.
The structure shown in FIG. 13A contains a substrate 21 having formed thereon a dielectric layer 35 containing a multilayer film formed by accumulating films 33 and 34 alternately, which are formed with two kinds of dielectric materials having different refractive indices n, by which only light having a particular wavelength λ0 is selectively reflected through the interference effect. The thickness L of the respective layers is L=iλ0/4n, where n (n1 or n2) represents the refractive index, and i represents a positive integer, which is 1 herein.
FIG. 14 is a graph showing calculation results of reflection spectrum of the dielectric layer 35 estimated by the effective Fresnel coefficient method. In the calculation, it is assumed that the refractive index n1 of one of the dielectric materials is 1.2, the refractive index n2 of the other dielectric material is 1.8, and λ0 is 520 nm, and the calculation is made for the number of accumulation j of from 1 to 5, respectively. It is understood from FIG. 14 that when the number of accumulation j is increased, the reflectivity R at λ0 is increased, and when the dielectric layers are accumulated by 5 layers, the reflectivity R reaches 90% or more. However, the full width at half maximum of the reflectivity at λ0 is as large as about 200 nm.
In the structure shown in FIG. 13B, spherical fine particles 9 having been classified in particle diameter are arranged on a substrate 21, and plural layers 40 formed with the fine particles are accumulated.
FIG. 15 is a graph showing a reflection spectrum of a film formed by accumulating layers of silica fine particle having a particle diameter of 280 nm formed by the self-organization described later. The reflection spectrum is measured in such a manner that white light is perpendicularly incident on the surface of the fine particle layer, and the spectrum of the light reflected perpendicularly to the surface of the layer is measured. It is understood from FIG. 15 that the reflectivity becomes maximum at a wavelength of 625 nm with a relatively high reflectivity of 54%, and the full width at half maximum of the peak is as narrow as about 30 nm.
It has been known that the Bragg's law is effective on interference of an X-ray by atoms or molecules forming a crystal. It has also been known that light is generally liable to be reflected by a periodical arrangement structure of fine particles repeated at a distance (pitch) nearly equal to the wavelength thereof. Accordingly, assuming that the relationship similar to conditions of the Bragg's law is effective in reflection of visible light on a silica fine particle layer, the wavelength λ0 of light that is most liable to be reflected and the distance (pitch) d of the fine particle layers have the relationship kλ0=2n3d, where n3 represents a mode refractive index of the constituent material of the fine particles, and k represents a positive integer.
The arrangement structure of fine particles is not conclusive, but the most frequent arrangement structure of rigid spheres, such as silica fine particles, is the close packed structure. The close packed structure includes the cubic close packed structure, in which three fine particle layers (A layer, B layer and C layer) having different arrangement positions of particles on the plane are repeated, and the hexagonal close packed structure, in which two fine particle layers (A layer and B layer) having different arrangement positions of particles on the plane are repeated. The distance (pitch) d between the adjacent two fine particle layers is common to the structures and has the following relationship to the diameter D of the fine particles, d=(2×3)1/2D/3.
It is assumed that silica fine particles exhibit the close packed structure, and in the aforementioned two equations, substitutions of the particle diameter of the silica fine particles of 280 nm for D, the mode refractive index of the silica fine particles of 1.36 n3, and 1 for k provide the most reflexible wavelength λ0 of 622 nm, which well agrees with the actual value λ0 of 624.5 nm.
It is considered from the aforementioned discussions that the silica fine particle layer formed by the self-assemble forming manner shown in FIG. 13B exhibits, in at least part thereof, a periodical particle arrangement having the close packed structure, which is the main factor of the reflection of light with a central wavelength of 624.5 nm.
From the practical standpoint, it is important, rather than the structure itself, that silica fine particles form such a reflective layer that has a reflection spectrum characteristic having a sharp peak with a narrow full width at half maximum shown in FIG. 15 owing to the structure formed by the self-assemble forming manner.
According to the model calculation where the close packed structure is simplified, a reflective layer having a sharp peak near 625 nm with a full width at half maximum of about 30 nm is formed by using silica fine particles having a refractive index of 1.36 and a particle diameter of 280 nm, which well agrees with the experimental value. According to the calculation, light having a wavelength of 625 nm thus being incident penetrates only to the eighth to fifteenth layer from the surface, but the most of the light is reflected by these layers to turn over the traveling direction thereof, and in particular, the boundary of reflection is around the eleventh layer. It is understood from the result that it is sufficient to provide about eleven layers for forming a light reflective layer with silica fine particles.
While the layer for reflecting red light (wavelength: 625 nm) has been described, layers for reflecting green light and blue light can be similarly produced. It is considered from the aforementioned discussions that the diameter of fine particles is proportional to the wavelength of light to be reflected, and therefore, fine particles having an appropriate diameter are selected depending on the wavelength of light to be reflected. That is, silica fine particles having a particle diameter of 235 nm may be used for green light (wavelength: 525 nm), and silica fine particles having a diameter of 212 nm may be used for blue light (wavelength: 475 nm).
FIG. 16A is a cross sectional view showing a basic structure of a reflective type screen reflecting only light of the three primary colors derived from the aforementioned discussions. Silica fine particles having a particle diameter of 280 nm are accumulated by 11 layers as a fine particle layer 2 for reflecting red light, silica fine particles having a particle diameter of 234.5 nm are accumulated thereon by 11 layers as a fine particle layer 3 for reflecting green light, and silica fine particles having a particle diameter of 212 nm are further accumulated thereon by 11 layers as a fine particle layer 4 for reflecting blue light, so as to form such a reflective layer that reflects only light of the three primary colors but transmits light of other wavelengths.
FIG. 16B is a cross sectional view showing a structure where the accumulation order of the reflective layers 2 to 4 is inverted. Since light having a shorter wavelength is liable to be scattered, it is preferred that the blue light reflective layer 4 is as the uppermost layer as in the arrangement of FIG. 16A for reducing scattered light. However, the fine particle layer having a smaller particle diameter is laid over the fine particle layer having a larger particle diameter in the arrangement of FIG. 16A, and therefore, the particle arrangement of the upper layer is liable to be disturbed by receiving influence of the particle arrangement of the lower layer. In the arrangement of FIG. 16B, on the other hand, the fine particle layer having a larger particle diameter is laid over the fine particle layer having a smaller particle diameter, and therefore, the particle arrangement of the upper layer is relatively hard to receive influence of the particle arrangement of the lower layer. Therefore, the arrangement of FIG. 16B is preferred for forming a regular particle arrangement.
A visible light absorber 1 absorbing visible light is used as a substrate. Specifically, for example, a black substrate formed with carbon is preferably used. In the case where the thickness of the visible light absorbing material 1 is increased, the mechanical strength thereof is increased, but the flexibility thereof is reduced. The thickness is preferably from 20 to 500 μm for balancing the mechanical strength and the flexibility, and for example, a thickness of about 50 μm is more preferred. A substrate having a thickness of about 50 μm provides a screen that is hardly broken but is easily wound owing to high flexibility. The area of the screen is appropriately selected depending on purposes.
The screen functions as a filter for selecting light depending on wavelength by itself according to the aforementioned mechanisms, and accordingly, the color reproducibility of an image is also improved by using the screen 30 owing to the following reasons. The spectral full width at half maximum of light of the three primary colors emitted from a CRT projector and a liquid crystal display projector is broad, as having been described. However, when the light is incident on the screen 30, only light of the three primary colors and the narrow wavelength regions in the vicinities thereof is selectively reflected by the reflective layer 32, but other light is wholly absorbed by the absorbing layer 31. Consequently, the light of the three primary colors thus reflected by the screen is improved in color purity with a narrow spectral full width at half maximum. Therefore, the color tone of the image formed by mixing light of the three primary colors is more accurately reproduced.
FIG. 17 is a graph showing a chromaticity diagram demonstrating the fact that the color reproducibility of an image reproduced by a liquid crystal display (LCD) projector and a DLP (digital light processing) projector is improved by using the screen according to the prior invention.
In the projectors, the spectral full width at half maximum of light of the three primary colors is as large as from 60 to 100 nm to provide poor color purity since wavelength selection of the light of the three primary colors is attained by using color filters. Therefore, the color reproducible area is restricted upon projecting onto a white screen as shown in FIG. 17.
In the case where a screen according to the prior invention shown in FIGS. 16A and 16B is used, the spectral full width at half maximum of light of the three primary colors thus reflected by the screen is narrowed to about 30 nm, whereby the color reproducible area is enhanced as shown in FIG. 17.
Various methods have been reported as a method for accumulating a fine particle aggregate having fine particles that are three-dimensionally regularly and periodically arranged through aggregation of the fine particles by self-assemble forming manner, so as to form a fine particle layer constituting the red light reflective layer and the like (P. Jiang, et al., Chem. Mater., vol. 11, p. 2132 (1999), and Y. Xia, et al., Adv. Mater., vol. 12(10), p. 693 (2000)).
One example of the methods is a draw up method. In the draw up method, as shown in FIGS. 18A to 18D, for example, a fine particle dispersion 11 containing fine particles dispersed in a dispersion medium is put in a dispersion bath, into which a substrate 1 having good affinity with the fine particle is perpendicularly inserted, and then the substrate 1 is drawn up from the fine particle dispersion 11. Upon drawing up the substrate, an appropriate amount of the fine particle dispersion is transferred to the surface of the substrate. Thereafter, self-assembly of the fine particles occurs during evaporation of the dispersion medium from the fine particle dispersion thus transferred, so as to form a fine particle aggregate having fine particles regularly arranged on the substrate (K. Nagayama, J. Soc. Powder Technol. Japan, vol. 32, p. 476 (1995), J. D. Joannopoulos, Nature, vol. 414(15), p. 257 (2001), and Yong-Hong Ye, et al., Appl. Phys. Lett., vol. 78(1), p. 52 (2001)).
Another example of the methods is a spontaneous sedimentation method. In the spontaneous sedimentation method, as shown in FIG. 19, a fine particle dispersion 11 is prepared by using a dispersion medium 10 in the similar manner as in the draw up method, and then a substrate 1 is still stood at the bottom of the fine particle dispersion. Fine particles 9 gradually sediment on the substrate owing to the weight thereof to form a fine particle aggregate having fine particles regularly arranged (H. Miguez, et al., Adv. Mater., vol. 10(6), p. 480 (1998)). Accordingly, the spherical fine particles 9, such as silica fine particles, are gradually accumulated on the substrate 1 from the dispersion by spontaneous sedimentation by gravity and by reduction of the amount of the dispersion medium by evaporation.
In still another example of the methods, a microcell formed by sandwiching a spacer larger than fine particles is perpendicularly inserted into a fine particle dispersion and still stood therein. The fine particle dispersion is charged into the cell by capillarity. Thereafter, self-assembly of the fine particles occurs during the process of evaporating the dispersion medium from the fine particle dispersion, so as to form a fine particle aggregate having fine particles regularly arranged in the cell (B. Gates, D. Qin and Y. Xia, Adv. Mater., vol. 11, p. 466 (1999)).
FIG. 20 is a schematic cross sectional view of a practical reflective type screen having a light diffusion film as a light diffusing layer 7 provided as the uppermost layer on a fine particle layer 40 formed according to the aforementioned manner. The light diffusion film 7 may be replaced, for example, by a microlens film having microlens arrays two-dimensionally formed on the surface thereof.
The reflective type screen selectively reflects only light of the three primary colors, red, green and blue, of a projector by utilizing the Bragg reflection of a photonic crystal obtained by regularly arranging fine particles and also makes black color pure by absorbing the external light by the substrate. The screen necessarily has a mechanical strength in a certain extent. In the case where the screen is wound, it necessarily has a strength against bending, tensility and compression, and also it necessarily has such a strength that withstands abrasion and press on the surface within the range where the screen is ordinarily used. Therefore, it is necessary to provide a photonic crystal that has the mechanical strength.
In some cases, the screen requires a light diffusing layer 7, such as a diffusion film on the surface as shown in FIG. 20. The light diffusing layer 7 is provided to relax the directivity to improve the viewing angle characteristics, whereby an image can be viewed in an oblique direction, and to avoid a hot spot, which is such a phenomenon that a light source of a projector is directly viewed by mirror reflection.
However, the fine particle accumulated layer is poor in mechanical strength against bending stress, tensility and compression stress to cause a problem in reliability due to peeling and breakage of the crystal. In the case shown in FIG. 21 where a protective film 6 is provided by coating an ordinary polymer material on a fine particle accumulated layer 40 in order to solve the problem, there are many cases where a large amount of the polymer 6A penetrates into gaps among the fine particles. In the case where the polymer penetrates into gaps among the fine particles constituting the photonic crystal, the optical characteristics, such as reflection characteristics and diffraction characteristics, thereof are influenced by displacing the difference in refractive index between the fine particles and the air by the difference in refractive index between the fine particles and the polymer.
A diffusion film 7 is provided on the photonic crystal layer 40 for improving the viewing angle characteristics, such as reduction of the directionality of the screen, and for avoiding the hot spot. In the case shown in FIG. 20 where an air layer 50 having a low refractive index is present between the photonic crystal 4 and the diffusion film 7, the external light is reflected on the back surface of the diffusion film to deteriorate purity of black color. Results obtained by simulating the phenomenon by the FDTD method (finite difference time domain method) are shown in FIGS. 22 to 25 and described below.
In a model of the diffusion film 7 shown in FIG. 22 where a bead 9 having a diameter of 5 μm is buried in a film 7 by half, a pulse of light 51 having a wavelength 520 nm is perpendicularly incident on the front surface of the film (plane wave). The refractive indices of the bead 9 and the film 7 are both 1.6. A monitor 52 is provided to calculate the intensity of light passing through the monitor.
The calculation results in this case with lapse of time are shown in FIGS. 23A to 24I. It is understood from the figures that the light passing the film 7 is spread as a spherical wave (diffusibility). The major reflection wave appears twice. The primary reflection wave (FIG. 23D) mainly contains a component of light that is reflected on the surface of the particle and returns as a spherical wave. The secondary reflection wave is a component of light that is reflected on the back surface of the film and returns, which is a spherical wave (FIG. 24G) in the particle owing to the lens effect of the particle, but becomes a plane wave (FIGS. 24H and 24I) in the exterior thereof, and thus, light with directionality returns.
FIG. 25 is a graph showing the light intensity calculated at the position of the monitor 52 as the ordinate with respect to the product of time T and the velocity of light c as the abscissa. It is found from the results that the intensity of light reflected by the diffusion film is about 8% of the incident light, and about 62% thereof are the secondary reflection wave. Therefore, the reflection can be suppressed by about 38% if the gap between the diffusion film and the photonic crystal can be filled with a material having the similar refractive index.
Accordingly, it is suitable as shown in FIG. 26 that an ordinary polymer material is coated as an adhesive material layer 6B directly on a photonic crystal, and using it as an intermediate layer, a diffusion film 7 is closely adhered thereon. In this case, however, the polymer material 6B penetrates into the photonic crystal to exhibit adverse affect on the characteristics of the screen, such as decrease of the reflectivity of the three primary colors (Bragg reflection).
The discussions herein have been made mainly for a reflective type screen, but the same problem occurs in general optical functional elements using a photonic crystal. In particular, the same problem occurs in the case where a polymer material is coated directly on a photonic crystal for forming a protective film or a waveguide.